A weft prefeed and measurement apparatus may comprise, as disclosed in the U.S. Pat. Nos. 4,132,370 and 4,238,080, a tubular finger rigid with a rotary shaft having an axial channel drilled therein which enables the inlet of the weft and its passage through the tubular finger and which provides for the winding of the weft in turns about a drum which is rotationally fixed. During each cycle the shaft should carry out a predetermined number of rotations N so that the finger winds a length of weft of N turns corresponding to the width of the fabric about the drum.
If the weaving machine is designed to carry out a weft selection, it comprises at least two prefeed and measurement devices for the weft, which devices are coupled to rotary drive means. In addition, special devices must be provided in order to obtain the selective rotary drive of the shaft of one or other of the prefeed and measurement devices. This selective rotary drive should satisfy several conditions:
It should operate synchronously with the overall operation of the weaving machine.
It should be possible to control it in correspondence with the selection of the weft for weaving, such that a length of the correct stored weft is always available for insertion in accordance with the machine program.
Each time that a weft prefeed and measurement device is actuated, its shaft should perform a predetermined number of rotations in order to wind the number of turns of the thread corresponding to the width of the fabric, or a multiple of this number of turns in the case of several successive insertions of the same weft.
In addition, the tubular finger of each prefeed and measurement apparatus should occupy, at the beginning and each of each operating cycle, a predetermined angular position so that the turns may be suitably called up.
In order to satisfy these conditions, it has already been envisaged to use a drive device which may be disengaged and is interposed between a motor element which is caused to rotate in a permanent manner synchronously with the overall operation of the weaving machine, and an output shaft connected to the weft prefeed shaft, for example by means of a synchronous belt transmission. The main element of this device is a clutch-brake comprising a plurality of coaxial members whose coupling or disconnection are controlled by the admission of a pressurized fluid, in particular compressed air, the structure being such that certain pneumatic connections are only established between predetermined relative positions of the various coaxial members.
The clutch-brake of this device is relatively complex from the point of view of construction and does not correspond to any commercially available product. Both pneumatic circuits and electrical circuits, for the control of an electrovalve disposed at the pressurised fluid intake, are required for the operation and control of this clutch-brake. In addition, although this device automatically compensates coupling and braking errors, it does not ensure the precise positioning of the tubular finger of the prefeed apparatus at the beginning and end of each operating cycle. Finally it is necessary to provide a mechanical connection between the main shaft of the weaving machine and all the weft prefeed and measurement devices.